1. Field of the Invention
The present invention relates to an object, a method, or a manufacturing method. Further, the present invention relates to a process, a machine, manufacture, or a composition of matter. In particular, the present invention relates to, for example, a semiconductor, a semiconductor device, a display device, a light-emitting device, a lighting device, a power storage device, a memory device, or a processor. The present invention relates to a method for manufacturing a semiconductor, a semiconductor device, a display device, a light-emitting device, a lighting device, a power storage device, a memory device, or a processor. The present invention relates to a method for driving a semiconductor device, a display device, a light-emitting device, a lighting device, a power storage device, a memory device, or a processor.
In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. A display device, a light-emitting device, a lighting device, an electro-optical device, a semiconductor circuit, and an electronic device include a semiconductor device in some cases.
2. Description of the Related Art
A technique for forming a transistor by using a semiconductor over a substrate having an insulating surface has attracted attention. The transistor is applied to a wide range of semiconductor devices such as an integrated circuit and a display device. Silicon is known as a semiconductor applicable to a transistor.
As silicon which is used as a semiconductor of a transistor, either amorphous silicon, polycrystalline silicon, or single crystal silicon is used depending on the purpose. For example, in the case of a transistor included in a large-sized display device, it is preferred to use an amorphous silicon film, which can be formed using the established technique for forming a film on a large-sized substrate. On the other hand, in the case of a transistor included in a high-performance display device where driver circuits and a pixel circuit are formed over one substrate, it is preferred to use polycrystalline silicon, which can be used for forming a transistor having high field-effect mobility. In the case of a transistor included in an integrated circuit and the like, it is preferred to use single crystal silicon, which can be used for forming a transistor having further high field-effect mobility. As a method for forming polycrystalline silicon, high-temperature heat treatment or laser light treatment which is performed on amorphous silicon has been known.
In recent years, an oxide semiconductor has attracted attention. An oxide semiconductor can be formed by a sputtering method or the like, and thus can be used for a semiconductor of a transistor in a large display device. A transistor including an oxide semiconductor has high field-effect mobility; therefore, a high-performance display device where driver circuits and a pixel circuit are formed over the same substrate can be obtained. In addition, there is an advantage that capital investment can be reduced because part of production equipment for a transistor including amorphous silicon can be retrofitted and utilized.
A transistor including an oxide semiconductor is known to have extremely low leakage current in an off state. For example, a CPU or the like with low-power consumption utilizing the low leakage current of the transistor including an oxide semiconductor is disclosed (see Patent Document 1). In the case where a transistor including an oxide semiconductor is used for an integrated circuit such as a CPU, it is preferable to downsize transistors to be integrated.
In a highly integrated semiconductor device, influence of parasitic capacitance generated owing to overlap of a wiring and an electrode cannot be ignored in some cases. A self-aligned top-gate structure transistor using an oxide semiconductor is described (see Patent Document 2). Patent Document 3 discloses that even when an offset region is provided in a transistor, the transistor can have excellent electric characteristics by making electrons enter a semiconductor from a conductor electrode. With use of a technique disclosed in Patent Document 2 or 3, parasitic capacitance generated due to overlap of a wiring and an electrode can be reduced.
It is also disclosed that a transistor having a high field-effect mobility can be obtained by a well potential formed using an active layer including a semiconductor (see Patent Document 4).